1. Field of the Invention
The present invention relates to an image bearing material, an electrophotographic photoreceptor using the image bearing material, and an image forming apparatus using the photoreceptor. More particularly, the present invention relates to an electrophotographic photoreceptor including a graft copolymer having a core/shell structure as an image bearing material, and an image forming apparatus using the photoreceptor.
2. Discussion of the Background
Electrophotographic image forming methods used for copiers, facsimile machines, laser printers, direct digital print-plate making machines etc. are well known. The image forming methods typically include the following processes:
(1) charging an electrophotographic photoreceptor (charging process);
(2) irradiating the charged photoreceptor with light to form an electrostatic latent image thereon (light irradiating process);
(3) developing the latent image with a toner to form a toner image thereon (developing process);
(4) optionally transferring the toner image on an intermediate transfer medium (transfer process);
(5) transferring the toner image onto a receiving material such as a transfer paper (transfer process);
(6) fixing the toner image to form a fixed toner image (fixing process); and
(7) cleaning the surface of the photoreceptor (cleaning process).
Until now, photoreceptors in which an inorganic material such as selenium, a selenium alloy or an amorphous silicone is formed on an electroconductive substrate as a photosensitive layer, or photoreceptors having a photosensitive layer in which an inorganic photoconductive material such as zinc oxide, cadmium sulfide or the like is dispersed in a binder resin have been used as photoreceptors for the electrophotographic image forming apparatus. Currently, photoreceptors using organic photosensitive materials are widely used because of having the following advantages over the above-mentioned photoreceptors:
(1) manufacturing costs are relatively low;
(2) it is easy to design a photoreceptor having a desired property; and
(3) hardly causing environmental pollution.
As the organic photoreceptors, the following photoreceptors are known:
(1) photoreceptors having a photosensitive layer including a photosensitive resin such as polyvinyl carbaozole (PVK) or the like material;
(2) photoreceptors having a photosensitive layer including a charge transfer complex such as polyvinyl carbaozole (PVK)/trinitrofluorenone (TNF) or the like material;
(3) photoreceptors having a photosensitive layer including a pigment, such as phthalocyanine or the like, dispersed in a binder resin; and
(4) photoreceptors having a functionally-separated photosensitive layer using a charge generation material and a charge transport material.
Among these organic photoreceptors, the photoreceptors having a functionally-separated photosensitive layer especially attract attention now.
The mechanism of forming an electrostatic latent image in a functionally-separated photosensitive layer having a charge generation layer and a charge transport layer is as follows:
(1) when the photosensitive layer is exposed to light after the layer is charged, the light passes through the transparent charge transport layer and then reaches the charge generation layer;
(2) the light is absorbed by the charge generation material included in the charge generation layer;
(3) the charge generation material generates a charge carrier such as electrons and positive holes;
(4) the charge carrier is injected to the charge transport layer;
(5) the charge carrier is transported through the charge transport layer due to the electric field formed by the charging;
(6) the charge carrier finally reaches the surface of the photosensitive layer and neutralizes the charge thereon, resulting in formation of an electrostatic latent image.
In the functionally-separated photoreceptors, a combination of a charge transport material mainly absorbing light having a wavelength in an ultraviolet region and a charge generation material mainly absorbing light having a wavelength in a visible region is typically used.
Until now, low molecular weight charge transport materials have been mainly developed and used. Since low molecular weight compounds have poor film forming ability when used alone, the compounds are typically used together with an inactive polymer while being dispersed therein. The charge transport layer constituted of a low molecular weight charge transport material and an inactive polymer is typically soft. Therefore, the photoreceptor having such a charge transport layer has a drawback such that when repeatedly used in electrophotographic processes, the charge transport layer is easily abraded due to the mechanical stresses applied thereto in the developing process and cleaning process.
In addition, such a charge transport layer has a relatively low charge mobility, and therefore it is difficult to design a high speed electrophotographic image forming apparatus or a small-sized image forming apparatus. This is because the concentration of the low molecular weight charge transport material is not greater than 50% in the charge transport layer. When the concentration of the low molecular weight charge transport material is greater than 50%, the film formability and abrasion resistance of the resultant charge transport layer seriously deteriorate.
In attempting to solve such a problem, Japanese Laid-Open Patent Publication (hereinafter xe2x80x9cJOPxe2x80x9d) No. 5-216250 discloses a photoreceptor using an improved binder resin. However, the abrasion resistance cannot dramatically improved because a charge transport material is included in the layer in an amount of about 50%.
In addition, JOPs Nos. 51-73888, 54-8527, 54-11737, 56-150749, 57-78402, 63-285552, 64-1728, 64-13061, 64-19049, 3-50555, 4-175337, 4-225014, 4-230767, 5-232727, and 5-310904 have disclosed polymers having a charge transport ability. The life of a photoreceptor can be extended to some extent by using such a charge transport polymer. However, the life is not satisfactory because when used in an image forming apparatus, the photoreceptor has to be changed at regular intervals due to the abrasion of the photosensitive layer.
JOPs Nos. 07-295248, 07-301936 and 08-082940 have disclosed photoreceptors in which a fluorine-containing silicone oil is included in their surface layer to improve the cleaning property of the layer and the abrasion resistance of the photoreceptors. However, a fluorine-containing silicone oil tends to migrate to the surface portion of the surface layer. When the photoreceptor is repeatedly used, the surface portion is easily abraded, and therefore the effect of the silicone oil disappears. Therefore, the life of the photoreceptor cannot be not extended.
Further, photoreceptors in which a particulate filler is included in the photosensitive layer to improve their abrasion resistance have been disclosed. For example, a particulate silicone resin or fluorine-containing resin (JOP No. 63-65449) and a particulate melamine resin (JOP No. 60-177349) have been disclosed. In addition, JOPs Nos. 02-143257, 02-144550, 07-128872 and 10-254160 have disclosed techniques in which a polyethylene powder (02-143257), a fluorine-containing powder (02-144550) or a particulate silicone oil (07-128872 and 10-254160) is included in a surface layer to decrease the coefficient of friction of the surface layer and to improve the abrasion resistance as a result of improvement of the cleaning property of the surface layer.
In addition, techniques in which a crosslinked organic particulate material (JOP No. 2000-010322 and U.S. Pat. No. 5,998,072), or a methylsiloxane resin powder (JOP No. 08-190213) is included in a surface layer to decrease the coefficient of friction of the surface layer and to improve the abrasion resistance as a result of improvement of the cleaning property of the surface layer.
These techniques intend to improve the abrasion resistance of the photoreceptor by decreasing the coefficient of friction or surface energy of the surface of the photosensitive layer.
However, the photoreceptors have the following drawbacks:
(1) the resin powders and particulate resins typically have poor compatibility with a binder resin used in the surface layer, and therefore the resin powders are not dispersed well in the surface layer, resulting in formation of undesired images such as black spots image or white spots;
(2) when repeatedly used, the residual potential of the photoreceptor increases, resulting in formation of image having low image density or background development; and
(3) the transmission of light through the photosensitive layer is obstructed, resulting in deterioration of photosensitivity and charge transporting ability, and thereby undesired images having uneven image density tend to be produced.
Because of these reasons, a need exists for a photoreceptor having a high photosensitivity and good abrasion resistance.
Accordingly, an object of the present invention is to provide an image bearing material having good abrasion resistance which can be used for an electrophotographic photoreceptor, and an intermediate transfer belt, a fixing belt and the like medium for use in the image forming apparatus.
Another object of the present invention is to provide an electrophotographic photoreceptor having a combination of good photosensitivity and good abrasion resistance.
Yet another object of the present invention is to provide image forming apparatus which can produce images having good image qualities at a high speed without frequently changing a photoreceptor.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an image bearing material including a matrix resin and, within the matrix resin, a graft copolymer having a core/shell structure which includes a core including one or more polymers and a shell including a polymer which has a linear chain structure connected with the core and which is formed from one or more monomers. The shell may have a linear structure having a polar group. In addition, the shell may be constituted of two or more layers.
The core preferably includes a complex polymer in which two or more polymers are intertwined such that the polymers cannot be separated from the other and the shell is formed from one or more vinyl monomers. Alternatively, the core material may be a complex polymer including two or more polymers having network structures which invade each other.
The complex polymer preferably includes at least a polyorganosiloxane.
In another aspect of the present invention, an electrophotographic photoreceptor is provided which includes an electroconductive substrate, and a photosensitive layer formed on the substrate and including the image bearing material mentioned above. The core/shell graft polymer is preferably included in the surface layer of the photoreceptor. Alternatively the photosensitive layer may include a charge generation layer and a charge transport layer, wherein the core/shell graft polymer is included in the surface layer.
In yet another aspect of the present invention, an image forming apparatus is provided which includes the electrophotographic photoreceptor of the present invention as an image bearing member.
In a further aspect of the present invention, a process cartridge is provided which can be attached to or detached from an electrophotographic image forming apparatus and which includes the photoreceptor of the present invention and at least a device selected from the group consisting of a charging device, a light irradiating device, a developing device, a transfer device and a cleaning device.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.